Pressure induced surface wetting for enhanced spreading and controlled filament size

ABSTRACT

A roller includes a cylindrical outer surface of a hydrophobic material, an inner core of a hydrophilic material, and an inhomogeneous geometric pattern of grooves in the surface that expose the hydrophilic material. A method of manufacturing a roller, includes providing a cylindrical core of a hydrophilic material, covering the cylindrical core with a hydrophobic surface, creating grooves in the hydrophobic surface to form a geometrically inhomogeneous pattern of the hydrophilic material. A method of manufacturing a roller, includes forming a pattern of geometrically inhomogeneous grooves on a hydrophobic core, functionalizing the surface to make the surface hydrophilic, and removing a portion of a top layer of the hydrophobic core to expose the hydrophobic core, leaving hydrophilic grooves.

RELATED APPLICATIONS

This application is related to the following US applications andpatents:

US Patent Publication No. US2015011947, “Method of Creating an Aerosolby Stretching Filaments Between Two Diverging Surfaces,”;

US Patent Publication No. US20150343477, “System for Creating Aerosolsby Stretching Filaments,”;

US Patent Publication No. US20150115057, “System for Creating Aerosolsby Stretching Filaments,”;

US Patent Publication No. US20150210009, “Spray Deposition System,”;

US Patent Publication No. US20150343468, “System for Creating Aerosolsby Stretching Filaments,”;

U.S. Pat. No. 9,257,056, “System for Creating Aerosols by StretchingFilaments,”;

US Patent Publication No. 20160175856, “Spray Deposition System,”;

U.S. patent application Ser. No. 14/575,922, “System for CreatingAerosols by Stretching Filaments,”;

U.S. patent application Ser. No. 15/001,408, “System Using AerosolGeneration and Selective Charging,”;

U.S. patent application Ser. No. 15/001,452, “Method Using AerosolGeneration and Selective Charging,”; and

U.S. patent application Ser. No. 15/651,195, “Central Fed Roller forFilament Extension Atomizer,”.

TECHNICAL FIELD

This disclosure relates to filament extension atomizer systems, moreparticularly to rollers used in these systems.

BACKGROUND

Palo Alto Research Center, Inc. (“PARC”) has developed a filamentextension atomizer system that generates aerosols from liquids. Thesystem generally involves stretching a liquid filament between twodiverging surfaces until the filament breaks up into a spray ofdroplets. In some versions of the system, the fluid input to the systeminvolves doctor blades and the pressure formed between the two surfaces.In one version, the two surfaces are rollers and the rollers form a nipbetween them to distribute the fluid.

Typically, for most fluids this is very effective. However, for fluidswith relatively high surface tension, and therefore having large contactangles on a flat and smooth surface, spreading the fluid uniformly in acontrolled manner on rapidly moving surfaces becomes problematic. Thespreading can typically be enhanced by using more hydrophilic materials,such as plastics with polar monomers or metal oxides, which lower thecontact angle. Although even this approach often fails to generate idealsurface wetting for generating a consistent spray from high surfacetension fluids.

SUMMARY

An embodiment consists of a roller including a cylindrical outer surfaceof a hydrophobic material, an inner core of a hydrophilic material, andan inhomogeneous geometric pattern of grooves in the surface that exposethe hydrophilic material.

An embodiment consists of a method of manufacturing a roller includingproviding a cylindrical core of a hydrophilic material, covering thecylindrical core with a hydrophobic surface, creating grooves in thehydrophobic surface to form a geometrically inhomogeneous pattern of thehydrophilic material.

An embodiment consists of a method of manufacturing a roller includingforming a pattern of geometrically inhomogeneous grooves on ahydrophobic core, functionalizing the surface to make the surfacehydrophilic, and removing a portion of a top layer of the hydrophobiccore to expose the hydrophobic core, leaving hydrophilic grooves.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of radii involved in the development of a roller.

FIG. 2 shows an embodiment of a filament extension atomizer system.

FIG. 3 shows an embodiment of a pair of counter rotating rollers inwhich one is a roller with a helical groove.

FIG. 4 shows a diagram of a grooved roller.

FIG. 5-7 show different embodiments of grooved rollers.

FIGS. 8-9 show surface profile measurements for different rollerembodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As mentioned previously, issues arise with higher surface tensionliquids because they do not spread easily over most commerciallyavailable machinable materials, such as metals and thermoplastics. Highsurface tension fluids, particularly surfactant-free aqueous mixtures,tend to coalesce rather than spread and the contact angles of thesefluids may surpass 90°. Thus, manipulating the film thickness becomesincreasingly difficult as surface tension increases. Applying pressureto the surface of the roller with a doctoring blade results in fluidbeing trapped between the roller and blade, and eventually beingexpelled to the sides. When the interaction energy between the fluid andsurface is favorable, film thickness is modified by altering the flowrate from the feed with the doctoring blade in contact. The fluidadheres to roller surface and can pass through the doctoring bladebecause of the lower contact angle.

The use of high surface energy plastic rollers provides lower contactangles, but is often insufficient to spread the fluid with a doctorblade. Embodiments here rely on capillary pressure by using patternedgrooves on the surface, which generates strong fluid adhesion indiscrete, hydrophilic, regions of the roller. Varying the groovedimensions also allows control of the filament size which willultimately affect the size of the droplets in the spray. The fluids canbe strictly confined to the grooves by coating the ridges with ahydrophobic material, and having them separated by ridges less thanthree times the size of the grooves. This effectively narrows thedroplet size distribution by generating filaments in discrete sizes.

These rollers are fabricated by using a hydrophilic core such aspolyetherimide, and in some embodiments may involve coating it with ahydrophobic material such as PTFE, Teflon, etc., only a few micronsthick. The channels are then created using a turn finish of a desiredradius, R_(a), depending on desired filament size. The capillarypressure in the channels spontaneously draws fluid into them, while thehydrophobic ridges repel fluid into the hydrophilic channels. Thecapillary pressure is calculated using the Laplace equation:

${\Delta\; P} = {\sigma( {\frac{1}{R_{1}} + \frac{1}{R_{2}}} )}$where R₁ and R₂ are the radii of curvature of the surface and σ is thesurface tension.

$R_{1} = {R_{m} = {\frac{R}{\cos( {\theta + \vartheta} )}.}}$R_(m) is substituted for R₁ which is shown in FIG. 1. The angle θ is thecontact angle of the fluid on the surface, and ϑ is the angle bisectingthe capillary. For situations where R₂>>R₁, the term 1/R₂ is omitted,which gives rise to:

${\Delta\; P} = \frac{\cos( {\theta + \vartheta} )}{R}$

FIG. 2 shows a filament extension atomizer system. Two counter rotatingrollers 100 and 102 stretch a fluid 106 that flows through the nip 104.The nip 104 is the space between the two rollers 100, 102 into which thefluid is drawn when the rollers 100, 102 counter-rotate. As shown in theexploded view of the diagram, the fluid 106 pools at an upstream side ofthe nip 104 and moves through the nip 104 as the rollers rotate.

On a downstream side 108 of the nip 104, the fluid stretches between thesurfaces of the two rollers 100, 102 into a fluid filament 110. As therollers 100, 102 counter-rotate, the surfaces of the rollers 100, 102 towhich the fluid filament 110 adheres remains the same, but the spacebetween such surface is greater. The fluid filament 112 grows longer andthinner as the surfaces of the rollers 100, 102 rotate away from eachother. When the fluid filament 112 reaches a point of the liquid bridgeit becomes unstable at this point, this is also the capillary break-uppoint for the fluid filament 112. The fluid filament 112 breaks up intoseveral droplets 114 and leaves excess fluid 116 behind on each of theroller's surface. The excess fluid 116 retracts to the surface of itsrespective roller and can be part of the fluid that pools and movesthrough the nip on the next rotation of the rollers. The process can berepeated to provide a continuous mist.

To effect better spreading of high surface tension fluids, the surfaceof the rollers is geometrically inhomogeneous. These inhomogeneouspatterns can consist of regular geometric shape patterns, some examplesinclude but not limited to helical grooves, circular grooves, dimples ofvarious shapes or cross hatch patterns. As the term is used here, ahelical groove is one continuous groove that spirals around the roller.This is in contrast with a series of individual circular grooves, thatare all self-contained circles.

It is manufactured typically by turning the roller while a tool of somekind presses into the roller and traverses it while it spins. There areseveral other manufacturing methods to fabricate inhomogeneous geometricpatterns on roller surfaces which are all applicable to control thespreading of high surface tension fluids and tailoring the fluidfilament size, thereby the mist characteristics. These methods includeproviding a cylindrical core of a hydrophilic material, covering thecylindrical core with a hydrophobic surface, and creating grooves in thehydrophobic surface to form a geometrically inhomogeneous pattern of thehydrophilic material.

Covering the cylindrical core with a hydrophobic surface may result fromone of many processes. In one embodiment, including casting thehydrophobic material over the hydrophilic core, then hardening thehydrophobic material. In another embodiment, coating the hydrophobicsurface comprises spray coating the hydrophilic core prior to creatingthe grooves. The process may also involve plasma treating thehydrophilic core and applying a silylating agent to the surface. Inanother embodiment, a polymer is bound to the surface through chemicallinkage, which may be covalent, ionic, dative or through chelation, toform brushes or chains. This would allow selection of a monomer toachieve a desired balance between the hydrophobic and hydrophilicproperties.

Forming the grooves may also be accomplished in one of several ways.These include turning the hydrophobic surface on a lathe, sand blastingthe hydrophobic surface, and etching, either chemically or with a laser,the hydrophobic surface.

In an alternative embodiment, the process may occur in a reverse manner.A hydrophobic core is first machined, sand-blasted, or laser etched to aselected texture. The surface is functionalized to make it hydrophilic.The small fraction of the top layer, or ridges, is removed in order toexpose the original hydrophobic surface, leaving the hydrophilic groovesuntouched.

FIG. 3 shows an embodiment of two counter rotating rollers. The toproller 102 has a smooth surface and the bottom roller 100 has apatterned surface. In this embodiment, the pattern is a helical grooveat 108 threads per inch (TPI). As the fluid is picked up by eitherroller, it spreads over the patterned roller in a more uniform layer,even with higher surface tension fluids. This allows for morereproducible filament and therefore droplet formation.

In one embodiment, the roller is formed by taking a hydrophilic core andcovering it with a hydrophobic material. The hydrophobic material isthen cut away by machining, leaving hydrophilic grooves surrounded byhydrophobic ridges. FIG. 4 shows an example of such a roller. The roller100 has hydrophilic grooves such as 206 and hydrophobic ridges 204. Oneapproach may involve a hydrophobic coating on the ridges to ensureretaining the high surface tension fluid in the grooves.

FIGS. 5-7 show different embodiments of the patterning on the surface ofthe roller. FIG. 5 shows a helically grooved roller having 108 TPI. FIG.6 shows a roller having a helically grooved roller having 64 turns perinch. FIG. 7 shows a roller with a different pattern. In thisembodiment, the surface of the roller has a cross hatching pattern.

A helical groove has several different parameters, including the turnradius, the turns per inch of the helix, and the other dimensions of thegroove, such the width and the depth. The embodiments here set out 108TPI, 64 TPI, grooves of 127 micrometer width, and rollers with turnradii of 12.5 and 6.3 micrometers. These provide specific examples, andshould not be construed to limit the applicability of the claims toother dimensions or parameters. FIGS. 8 and 9 show surface profiles fora 64 TPI roller and a 108 TPI roller. The roller has 127 micrometer widegrooves running the axis of the roller.

In this manner, higher surface tension fluids can be converted to aspray of droplets using a filament extension atomizer system. Thegrooved roller allows for better spreading of the fluid over one of apair of counter rotating rollers. The control of the dimensions of thegeometric inhomogeneous patterns, such as the depth and width of thegrooves noted as examples above, can also provide better control overthe size of the filaments, which in turn provides better control of thesize of the droplets.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A roller, comprising: a cylindrical outer surfaceof a hydrophobic material; an inner core of a hydrophilic materialwithin the outer surface; and a geometric pattern of grooves in theouter surface that expose portions of the inner core of the hydrophilicmaterial to allow the portions to accept fluid.
 2. The roller of claim1, wherein the hydrophilic material comprises polyetherimide.
 3. Theroller of claim 1, wherein a size of ridges between the grooves is lessthan three times a size of the grooves.
 4. The roller of claim 1,wherein the hydrophobic material comprises polytetrafluoroethylene. 5.The roller of claim 1, wherein the geometric pattern is a helical grooveand has a turned finish having a radius based upon a filament size. 6.The roller of claim 1, wherein the grooves have dimensions that resultin a capillary pressure of less than −100 Pascals for a selected liquid.7. A method of manufacturing a roller, comprising: providing acylindrical core of a hydrophilic material; covering the cylindricalcore with a hydrophobic surface; creating grooves in the hydrophobicsurface to expose the hydrophilic material in a geometrical pattern toallow the grooves to accept fluid.
 8. The method of claim 7, whereincovering the cylindrical core with a hydrophobic surface comprisesinserting the hydrophilic core inside a hydrophobic cylindrical surface.9. The method of claim 7, wherein covering the cylindrical core with ahydrophobic surface includes casting the hydrophobic material over thehydrophilic core, then hardening the hydrophobic material.
 10. Themethod of claim 7, wherein creating the hydrophobic surface comprisesspray coating the hydrophilic core with a hydrophobic material prior tocreating grooves.
 11. The method of claim 7, wherein creating groovescomprises one of turning the hydrophobic surface on a lathe, sandblasting the hydrophobic surface, laser ablation, and etching thesurface.
 12. The method of claim 7, wherein covering the cylindricalcore with a hydrophobic surface comprises casting the hydrophobicsurface over the cylindrical core.
 13. The method of claim 7, whereincovering the cylindrical core with a hydrophobic surface comprisesplasma treating the hydrophilic core and applying a silylating agent tothe hydrophobic surface prior to creating the grooves.
 14. The method ofclaim 7, wherein covering the cylindrical core with a hydrophobicsurface comprises grafting polymer chains or brushes onto thehydrophobic surface prior to creating the grooves.
 15. The method ofclaim 14, wherein grafting the polymer chains on the hydrophobic surfacecomprises selecting monomers to achieve a desired balance of hydrophobicand hydrophilic properties.
 16. A method of manufacturing a roller,comprising: forming a geometric pattern of grooves on a surface of ahydrophobic core; making the surface of the hydrophobic corehydrophilic; and removing a portion of the surface of the hydrophobiccore to expose the hydrophobic core, leaving hydrophilic grooves toallow the hydrophilic grooves to accept fluid.
 17. The method of 16,wherein forming the pattern of grooves comprises one of machining, sandblasting, chemically etching or laser etching the hydrophobic core.